U.S. agriculture officials say the dairy animal infected with mad cow disease was born before safeguards were put into place to prevent transmission of the deadly brain illness. The officials see this as a sign no animal has become sick since the precautions took effect.

Chief U.S. Agriculture Department veterinarian Ron DeHaven says documents from a Washington state dairy farm confirm Canadian records that the infected Holstein was born in Alberta, Canada in April 1997.

That is four months before the two countries banned the use of nerve tissue from cattle in cattle feed, the means by which mad cow disease is transmitted between animals. "The age of the animal is especially important in that it is a likely explanation as to how this animal would have become infected," said Mr. DeHaven.

Dr. DeHaven said the animal might have become sick from contaminated feed before the ban went into effect and before being shipped to the United States, although he said only genetic tests would prove this.

The agriculture official says this would indicate the feed ban has worked as intended to prevent transmission of mad cow disease, formally known as bovine spongiform encephalopathy, or BSE.

The infected cow had three offspring born in the United States. Dr. DeHaven says transmission from the mother to the calves is very unlikely, but the calves are nonetheless being isolated as a precaution. Twenty-nine countries have stopped importing U.S. beef since the mad cow case was announced last week. But Dr. DeHaven said the embargoes are based on public perception, not on the relevant science of the disease.

In an effort to reassure the world that American beef supplies are safe, he pointed out that surveillance of 20,000 head of cattle per year for the last two years has turned up only this single case. "By any stretch of the imagination, the U.S. cannot be considered at high risk for BSE, especially given our high level of surveillance over the recent past and the fact that only one case of the disease has been found here, and, further, that a single case appears not to have been even born in the United States at this point," he said.

Dr. DeHaven defended U.S. cattle inspection procedures, which do not test all the animals for the disease. Instead, inspectors test only animals that cannot walk, like the one in Washington state, or that exhibit symptoms of the brain-wasting illness. He says Harvard University research shows that this sampling method can detect BSE even at the very low rate of one case per million cattle.

A spokesman for President Bush says the U.S. safeguards against mad cow disease are strong, but adds that the government is looking at possible ways to strengthen them.